This invention relates to subsynchronous overcurrent relays. More particularly, this invention relates to relays for detecting and measuring sustained subsynchronous current in electrical power equipment such as in generator unit windings and the like.
It is known that subsynchronous oscillations resulting in subsynchronous current occur on a series compensated power system as a result of the interaction of the series inductive and capacitive components of the system. More specifically, the dynamic interaction of a power system and its associated turbine generator is some instances result in the sustaining of the subsynchronous oscillations. These oscillations occur most noticeably during system transient as a result of system faults, or as a result of series capacitor switchings. In many instances, these ocillations are of a transient nature and are quickly damped. However, it has been determined that when subsynchronous oscillations occur for a sustained period, and if the subsynchronous oscillations are close to a natural shaft frequency of associated equipment such as a generator unit, the oscillations are amplified and generator damage may result.
In the past, the subsynchronous oscillations were neither detected nor measured and any associated generating unit was typically only protected from sustained subsynchronous oscillations when a failure, such as a generator field lead electrical failure at shaft bore entrance occurred. An actual failure may result not only in substantial repair costs, but also in a power failure during the shutdown period required to repair the damaged equipment.
Prior art relays, such as differential relays, which measure the difference between input and output signals, were found unsuitable for subsynchronous current applications since they were typically designed to provide protection from faults which result in a change in magnitude of the synchronous current, i.e. the current at the fundamental frequency. Differential relays with harmonic restraints were also found unsuitable since they were only concerned with restraining the relay from being operated by the harmonic frequency currents, i.e. multiples of the synchronous current, typically occuring during transitory conditions.